The present invention relates in general to the control of pulverizers for coal fired steam generators and, in particular, to a new and useful method and apparatus for adjusting the relationship between the mass flow of coal through the pulverizer to the primary air flow of the pulverizer, according to changes in the properties of the coal, in order to maintain an optimum performance for the pulverizer.
In a steam generator which utilizes a pulverizer for supplying coal or other fuel thereto, a small portion of the air required for combustion is used to transport the coal to burners or other structures for burning the fuel in the steam generator. This is known as primary air. In direct fire systems, primary air is also used to dry the coal in the pulverizer. The remainder of the combustion air is introduced at the burner and is known as secondary air.
The current technique for the control of a pulverizer in a coal fired steam generator plant is achieved by the use of a "coordination curve" which relates the primary air flow to the pulverizer with the required mass flow of coal through the pulverizer. The coordination curve is based on the grindability of the raw coal and the outlet fineness required to achieve efficient combustion in the steam generator. Thus, this curve defines the maximum and minimum coal flow capabilities of the pulverizer under the above conditions. Primary air temperature is controlled by dampers to achieve the prescribed coal drying from a set outlet temperature of the pulverized coal/air mixture.
Normally, the design coordination curve is adjusted during commissioning or operation to reflect the actual fuel properties (e.g. coal grindability) encountered and a classifier is adjusted to provide the outlet fineness required in the pulverizer.
Some steam generating stations use more than one coal supply and do not reset or re-adjust the pulverizer when a change in coal supply occurs. Under these circumstances, the coordination curve used in the pulverizer control loop must be a comprise among the intended fuels. This results in less than optimum conditions for the pulverizer, affecting combustion efficiency, turndown capability and response rate during load changes.
U.S. Pat. No. 4,528,918 to Sato et al discloses a method of controlling combustion which is fueled by one or more pulverizers that are supplied with primary air which conveys pulverized coal to burners that are supplied with secondary air for burning the coal.
U.S. Pat. No. 4,518,123 to Tanaka et al discloses a method of controlling a pulverizer which utilizes a push blower on the input side of the pulverizer and a pull blower on the output side of the pulverizer.
U.S. Pat. No. 4,424,766 to Boyle discloses a fluidized bed combustor, which is capable of using a variety of different coals. No mechanisms are disclosed for varying the operating parameters of the equipment according to the type of coal utilized, however.
U.S. Pat. No. 4,116,388 to Trozzi discloses a pulverized fuel burner which utilizes primary air that is conveyed along with the pulverized fuel and secondary air which is independently supplied to the burner.
Currently, problems are encountered when variations in fuel source occur. These problems are more prevalent in overseas utilities where different fuel sources are normally used.